Endodontists use various types of instruments for cleaning and enlarging the root canals of the teeth. In a typical root canal procedure, an endodontist first makes an opening in the surface of the tooth to provide access to the interior. The endodontist then utilizes small instruments, such as hand-held files and reamers, to clean and enlarge the narrow, tapered root canals. In a conventional procedure, the endodontist fills the prepared root canals with gutta percha, which is a rubber-like substance, and then seals the tooth with protective cement. The endodontists may sometimes apply a crown to the tooth as a final step.
Typically, the endodontist uses a series of delicate, flexible files to clean out and shape the root canals. Each file includes a proximal end, typically including a handle to be gripped between the fingers of the endodontist, and a distal end or tip. A working length with a tissue-removing configuration, such as helical or non-helical flutes and cutting edges, is located between the proximal and distal ends. The endodontist uses files of increasingly larger diameter to sequentially increase the diameter of the root canal and achieve the desired diameter and shape.
Endodontic instruments of the desired type having helical flutes are conventionally fabricated by permanently twisting (also called torsioning) a rod of triangular, square, or rhomboid-shaped cross section. The angles formed between the surfaces form the cutting edges, which spiral along the working length of the instrument. Another method for manufacturing instruments of the described type having either helical or non-helical flutes is by a machining process wherein an instrument blank is moved past a rotating grinding wheel. The instrument blank is thereafter indexed and again moved past the grinding wheel, and these steps are repeated as many times as are necessary to form the instrument blank into the desired cross section. The flute grinding process produces a directional surface finish along the cutting axis, which can have a tendency to propagate early material failure and introduce machining stresses into the material.
Over the past several years, endodontic instruments having helical flutes have been manufactured by simultaneously grinding and twisting thin carbon steel or stainless steel rods or wires. Specifically, steel wire blanks are first ground to the desired cross sectional shape, such as square, triangular or rhomboid, and to the appropriate size and taper. The ground blank is then gripped at one end and spring loaded jaws are brought into contact with the ground portion of the blank. As the blank is rotated from the gripped end, the jaws are moved axially away from that end. The jaws therefore twist the rotating blank and form helical flutes into the blank. The longitudinal, ground edges of the blank form helical cutting edges on the file. The axial jaw speed, twisting speed and spring force are controlled to obtain the desired helical configuration.
Carbon and stainless steel instruments are generally stiff, which may lead to errors during root canal therapy. With the emergence of superelastic materials, such as nickel-titanium alloys, endodontic instrument manufacturers are now able to form endodontic root canal files and reamers with much more flexibility. This greatly assists the endodontist during use of the file or reamer in a root canal procedure. The use of superelastic material, however, causes some significant manufacturing concerns due to the tendency of the material to return to its original shape after the release of an applied force. File or reamer blanks manufactured of superelastic materials generally react in this manner to the conventional twisting methods employed for manufacturing carbon and stainless steel files and reamers. Moreover, if superelastic blanks are over-stressed, such as by being twisted too much during the fluting procedure, the material is subject to failure. For reasons such as these, current manufacturers of endodontic instruments may resort to grinding the helical profile directly into the superelastic blanks while applying no twisting forces to the blanks. These direct grinding methods tend to introduce stress into the material.
In U.S. Pat. No. 6,149,501, a method is provided for manufacturing superelastic endodontic instruments in which a blank is provided and maintained in the austenite phase, preferably above the austenite finish temperature (Af), at least prior to a twisting operation and, preferably, prior to and during the twisting operation. During the twisting operation, the material is converted from the austenite phase to the martensite phase by the stress applied during the twisting operation. Thus, the superelastic material undergoes stress-induced martensite transformation from a 100% austenite phase. For this method, high temperature tooling is required because the twisting operation is performed at a temperature above the Af temperature. The tooling and file blank are preferably submerged in a heated liquid, such as an oil or salt solution at a temperature of 500° C. or above, to bring the material to a 100% austenite phase. The heated liquids, however, are generally corrosive to the tooling.
In U.S. Pat. No. 6,783,438, a method is provided for manufacturing superelastic endodontic instruments in which prior to twisting, the superelastic material is brought to an annealed state comprising a phase structure that is a rhombohedral phase, a combination of an austenite phase and a martensite phase, a combination of a rhombohedral phase and an austenite phase, a combination of a rhombohedral phase and a martensite phase, or a combination of a rhombohedral phase, an austenite phase and a martensite phase. While in this annealed state, the instrument is twisted to form the helical flutes. While eliminating the need for high temperature tooling, a twisting apparatus is still needed to produce the desired configuration for the instrument, and the flutes may only be helical by virtue of the twisting operation.
With the above background in mind, there is a need for a method of fabricating endodontic instruments, such as files and reamers, that avoids the disadvantages described above for grinding and/or twisting techniques, that provide an instrument having a desired tissue-removing configuration, such as either helical or non-helical flutes, and for instruments that are flexible and highly resistant to torsional breakage. It would further be desirable to provide a method of manufacturing a wide variety of superelastic endodontic instruments that does not require high temperature tooling.
In addition to tissue-removing endodontic instruments, other superelastic dental instruments also suffer from the disadvantages of known machining techniques, such as high induced stresses in the machined material. Due to the small and precise configurations necessary for instruments that are used in the oral environment, such as orthodontic instruments and implants, the machining of configurations for dental instruments is particularly challenging. There is thus a need for a method of manufacturing a variety of superelastic dental instruments that avoids the disadvantages described above for machining techniques.